The hero officer walked into a grocery store in Mataró, Spain, and as surveillance video shows ... stumbled into the armed robbery in progress. Cameras caught the suspect walking an employee to the cash register at knifepoint.

Just as the suspect was cleaning out the register ... the cop walked into frame, sized up the situation and quickly sprung into action.

He casually put down his shopping bag and then subdued the suspect with some wrestling moves, taking him down to the ground.

While holding the guy in a chokehold, you see the cop rip the knife away from the suspect ... who he detained until backup arrived.